Officials suspect a fertiliser bomb was used in an explosion in Nairobi that wounded 33 people.

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Police say an improvised explosive device caused a blast that ripped through a building full of small shops in downtown Nairobi, with one official saying it may have been a fertiliser bomb.

The official, who spoke on condition of anonymity, told The Associated Press that the smell of ammonia at the scene of Monday's explosion on Moi Avenue indicated the possible presence of a fertiliser bomb, which is commonly made of ammonium nitrate and fuel oil.

The FBI joined the investigation into the attack that wounded 33 people, including a woman who blamed the blast on a "bearded man" who left behind a bag shortly before the detonation.

Kenya's police spokesman, Eric Kiraithe, said police concluded on Tuesday that the blast was caused by an IED. Kiraithe said officials couldn't yet definitely blame the Somali militant group al-Shabab, an Islamist group that has links to al-Qaeda.

Al-Shabab threatened in October to bring down Nairobi skyscrapers and bragged about its July 2010 bomb attacks in Kampala, Uganda, that killed 76 people. Al-Shabab issued the threat against Kenya after Kenyan troops moved into Somalia to attack al-Shabab fighters.

FBI agents helped analyse materials at the blast site at Kenya's request, said US Embassy spokesman John Haynes. A group of FBI agents could be seen at the site on Tuesday, sifting through debris and packing samples with members of Kenya's Anti-Terror Police Unit.

The intelligence firm IntelCenter said al-Shabab militants bragged about acting as journalists and conducting interviews of survivors after the blast, posing significant challenges to security forces and legitimate members of the media covering attacks.

The explosion sent dark smoke billowing out of a one-story building on the avenue named after Kenya's second president.